R help - Jun 2010 - how can I evaluate a formula passed as a string?

Hey everyone,

    I've been using 'R' long enough that I should have some idea of
what the
heck either   expression()  or eval()  are really ever useful for.  I come
across another instance where I WISH they would be useful, but I cannot get
them to work.

    Here is the crux of what I would like to do:

presume df looks like this
 A      B      C
===  ===  ==M      45   0
M      46   1
F       42   0
F       42   1
M      46   1

and I want to make it look like this, using the function below (allowing for
other options besides "count" or "median"):
  A     B    Count of C    Median of C
===  ===  ==========  ===========M     45         1                  0
M     46         2                  1
F      42         2                  0.5

blah <- function(df=df, dup="C",
aligns=c("A","B"),
dupFuns=c("length(unique(x)","median(x)", dupNames) {
### A function where it will "widen" the df based upon the
"aligns" columns,
taking care to summarize the "dup" column using the
"dupFuns"
    tmp <- aggregate(df[,dup], by=as.list(df[,aligns]), FUN=function(x) { *
eval(dupFuns[1])* } )
    names(tmp)[length(names(tmp)] <- dupNames[1]
    for (i in c(2:length(dupFuns))) {
        tmp <- merge(tmp, aggregate(df[,dup], by=as.list(df[,aligns]),
FUN=function(x) { *eval(dupFuns[i])* } ), all = TRUE)
        names(tmp)[length(names(tmp)] <- dupNames[i]
    }
}

    Everything in it works OK, except that the "eval" function
doesn't work
as it should.  How can I get, for instance, "length(unique(x))" (as a
character string) to become length(unique(x))  (a function to be evaluated
as shown above).

                          Thanks!
                               Mike



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On Thu, Jun 24, 2010 at 10:16 AM, Mike Williamson <this.is.mvw at
gmail.com> wrote:
> Hey everyone,
>
> ? ?I've been using 'R' long enough that I should have some idea
of what the
> heck either ? expression() ?or eval() ?are really ever useful for. ?I come
> across another instance where I WISH they would be useful, but I cannot get
> them to work.
Example: eval(parse(text = "3*3"))

Peter